This invention relates to a process of purifying cellulose ethers and, in particular, to a process for purifying cellulose ethers which do not exhibit a gel point in liquid water.
Water-soluble cellulose ethers such as methyl cellulose, hydroxypropylmethyl cellulose, hydroxyethyl cellulose, hydroxyethylmethyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose, and the like are widely used as thickeners, binders, drug additives and for many other uses. Such cellulose ethers are commonly prepared by reacting cellulose pulp with sodium hydroxide and an etherifying agent such as methylchloride, ethyleneoxide, propyleneoxide, chloroacetic acid and the like. Because many side reactions occur during the etherification reaction, the crude cellulose ether contains substantial amounts of impurities such as salt and various glycol ethers. It is usually necessary to remove these impurities before the cellulose ether is ready for use.
Since these impurities are generally water-soluble, it would be desirable simply to wash the crude cellulose ether to dissolve the impurities therefrom. Unfortunately, however, the cellulose ether is itself water-soluble and accordingly, water washing to remove impurities results in substantial loss of product as well.
Certain cellulose ethers such as methyl cellulose and hydroxypropylmethyl cellulose exhibit an inverse solubility in water with rising temperature. That is, as the temperature of the water increases, the cellulose ether becomes less and less soluble therein. For such cellulose ethers, there is typically a characteristic gel temperature above which the cellulose ether is not water-soluble. Such gelling cellulose ethers may be water washed by conducting the washing step above the gel temperature of the cellulose ether.
Unfortunately, other cellulose ethers such as hydroxyethyl cellulose and hydroxyethylmethyl cellulose do not exhibit such a gel point at any temperature below the boiling point of water. Accordingly, one cannot solve the washing problem simply by using hot water to conduct the washing. It is therefore necessary to employ some other means to purify these non-gelling cellulose ethers.
It is known to render such non-gelling cellulose ethers temporarily insoluble in water by crosslinking them with a dialdehyde such as glyoxal. For example, in U.S. Pat. Nos. 3,347,847 and 3,527,751 hydroxyalkyl cellulose is prepared as a suspension in isopropanol or an isopropanol/water azeotrope. The crude hydroxyalkyl cellulose is, while in the presence in said azeotrope, treated with acid and glyoxal to crosslink it for water washing.
Similarly, in U.S. Pat. No. 3,709,876, hydroxyethylmethyl cellulose is prepared in a dry process (i.e., without liquid diluent) and is subsequently treated with glyoxal and acid to render it temporarily water-insoluble for washing.
Unfortunately, however, these prior art processes have distinct disadvantages. For example, the process described in U.S. Pat. Nos. 3,347,847 and 3,527,751 both require the use of an organic solvent to prepare and wash the cellulose ether. Thus, this organic solvent must be later removed from the cellulose ether, which presents the common problems normally associated with solvent recovery.
The process of U.S. Pat. No. 3,709,876, while free of the disadvantages associated with the use of organic solvents, requires treating the cellulose ether with a relatively high (5 to 15 percent treatment level) amount of glyoxal to sufficiently crosslink the cellulose ether for washing. In addition, despite the glyoxal treatment, large amounts of the crude cellulose ether remain water-soluble and are subsequently lost during the water washing step. For these reasons, the efficiency of the process of U.S. Pat. No. 3,709,876 is far lower than desired.
Accordingly, it would be desirable to provide a process for purifying crude non-gelling cellulose ethers in which the use of organic solvents is not required and low amounts of the crude cellulose ether are dissolved or otherwise lost during the purification process.